How can parents know their child is developing on schedule?

Regular checkups. In the first two years of life, your child should be seeing his pediatrician very frequently for regular well child checkups. During each of those checkups, your doctor will assess your child's various developmental milestones to inform you of his progress. A great resource online from the aap can be found at http://www.Healthychildren.Org/english/ages-stages/pages/default.Aspx.
Read up on milestone. If you want to read up a little on normal developmental milestones, read: http://sparkspeds.Blogspot.Com/2011/07/major-speech-milestones.Html and http://sparkspeds.Blogspot.Com/2011/07/major-gross-motor-milestones.Html.